The preferred embodiment concerns methods to control a development process in an electrographic printer or copier in which at least one developer station inks a latent image on an intermediate carrier with toner of a predetermined color, wherein the toner draws a mixture of toner and carrier particles and fresh toner from a reservoir supplied to the mixture.
In printing of recording media with single color or multicolor toner images, latent images on an intermediate carrier are inked by a respective developer station per toner color used. In operation, the size of the inked area on the recording media can fluctuate significantly, which is expressed by the degree of areal coverage. Given operation with low areal coverage, the toner consumption is also correspondingly low, which can affect the print quality. Operating phases with low or extremely low areal coverage are typically designated as low take-out operation (LTO operation) or no take-out operation (NTO operation). Given a multicolor printing, the areal coverage per color component is to be taken into account. It can hereby occur that an LTO operation exists for one specific toner color while the areal coverage as a whole is high. For example, given what is known as highlight color operation in which only single image elements are highlighted in color on a black-and-white image, the toner consumption for this color can be very low in relation to black toner. Given full color applications, the toner consumption per color is generally greater. Fluctuations in the inking of individual colors due to different toner consumption here are distinctly perceived as color shift, however. The degraded print quality in LTO or NTO operation has multiple causes, wherein a certain toner waste is hereby significant. On the one hand, the triboelectric properties, and therefore the adhesion properties of the toner particles to the intermediate carrier, change as a consequence of the agitation of the mixture of toner particles and carrier particles (in general ferromagnetic carrier particles). On the other hand, the mixture can change in terms of its mechanical properties due to the interaction between carrier particles and toner particles, in particular in the agitation of the mixture. Both effects can lead to a measurement error in the registration of the toner concentration with the aid of a sensor, with the consequence that overall the toner concentration in the mixture drops in a toner concentration regulation system. The connection between low triboelectric adhesion and low toner concentration leads to a reduced inking on the intermediate carrier and/or to degraded conditions for the transfer of the toner particles from intermediate carrier to the recording media (paper, for example). The print quality can suffer as a whole under this. A precise measurement of the properties of the mixture made up of toner and carrier particles is technically impossible due to the complex mechanical and electrical behavior of the mixture; sensors for this are also not present. It therefore appears reasonable to develop characteristic values using which the status and the behavior of the mixture can be estimated.
Document U.S. Pat. No. 6,173,133 describes a toner concentration regulation in which compensation algorithms are used. In addition to the temperature compensation of a sensor measuring the toner concentration, parameters for compensation of the break-in (break-in compensation) of the developer mixture and a toner aging compensation (toner age compensation) are also implemented.
Additional documents of the prior art are U.S. Pat. Nos. 6,047,142, 6,871,029, 7,079,794 and 7,085,506.
A method and a device to adjust the toner concentration in the developer station of an electrophotographic printer or copier is also known from WO 2004/012015 A1.
Document U.S. Pat. No. 4,614,165 describes a method and a developer device, in which method or in which developer device the entire mixture comprising toner and carrier particles is exchanged when toner and carrier have reached a predetermined age. An aging rate is determined that depends on the number of generated copies. Toner and carrier are then exchanged depending on this aging rate.
A method to calculate the toner age and the carrier age for printer diagnosis is known from document EP 1 951 841 A. A maximum toner age is stored in a memory. The current toner concentration in the developer mixture is measured, and the consumption rate for the developer and the toner is calculated from this. The toner age is determined on the basis of the toner concentration and the consumption of developer. A print job is interrupted if the maximum toner age is reached.